In-air ultrasonic rangefinding and angle estimation

ABSTRACT

An apparatus for determining location of a moveable object in relation to an input device includes an array of one or more piezoelectric micromachined ultrasonic transducer (pMUT) elements and a processor. The array is formed from a common substrate. The one or more pMUT elements include one or more transmitters and one or more receivers. The processor configured to determine a location of a moveable object in relation to an input device using sound waves that are emitted from the one or more transmitters, reflected from the moveable object, and received by the one or more receivers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/204,917 filed on Mar. 11, 2014, incorporated herein by reference inits entirety, which claims priority to, and the benefit of, U.S.provisional patent application Ser. No. 61/776,403 filed on Mar. 11,2013, incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED

Not Applicable

INCORPORATION-BY-REFERENCE OF COMPUTER PROGRAM APPENDIX

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains generally to a sensor system, and moreparticularly to a system which can measure the range and direction toobjects that are located in front of the sensor system.

2. Description of Related Art

Alternatives to the keyboard and mouse are being rapidly deployed incomputers, smartphones, and tablets. The touch screen interface hasgained traction in all three markets and dominates the smartphone andtablet market. The touch screen allows intuitive interfaces based onsoftware buttons to be used and is especially ideal for small screenswhere the user's hand can easily traverse the entire screen. However,there are many situations where a touch screen is inappropriate or couldbe complemented by an interface that does not require the user to touchthe screen.

Optical 3D imagers for gesture recognition suffer from large size andhigh power consumption. Their performance depends on ambientillumination and they generally cannot operate in sunlight. For example,the Microsoft Kinect 3D imaging system consumes 12 watts, has a volumeof over 675 cm³, and cannot operate in sunlight. These factors prohibituse in portable electronics.

Ultrasound transmitters and receivers have used continuous wave signalsto calculate the impulse response of a channel and to extract from thisthe user's gestures. However, use of continuous wave signals issusceptible to multipath interference, and requires extremely highdynamic range in the receive electronics. Multipath interference ariseswhen transmitted waves reflect off several targets and arrive back atthe receiver. If the waves have a slightly different path length, theycan combine destructively, thereby canceling the return signal. The highdynamic range requirement arises because the ultrasound wave isattenuated as it travels to the target and back. When the wave returns,it is many times smaller than the transmitted signal. Generally there issignificant leakage from the transmitter to the receiver, so thereceiver must detect the faint echo in the presence of a largefeedthrough component from the transmitter. This requires large dynamicrange.

For gesture input to portable devices, it is desirable to build a systemthat has <1 cm³ size, <10 mW power consumption, and which can be used ina variety of environmental conditions. This invention provides anenabling technology to accomplish these goals.

BRIEF SUMMARY OF THE INVENTION

An aspect of the present invention is to provide a system which canmeasure the range and direction to objects that are located in front ofthe sensor system. One application is gesture control for computerinterfaces. The system may comprise a gesture recognition system thatcan be used to track a user's hand or other parts of the body andtranslate the user's movements (e.g. position and motion) into inputcommands to the device.

By way of example, and not of limitation, the present invention usesultrasound waves to measure the range and angle to targets. In oneembodiment, an ultrasound wave is emitted from one or more ultrasoundtransceivers, and one or more ultrasound transceivers are used to detectthe echo. Time diversity in the transmitted signal allows thetime-of-flight to be calculated which is used to infer the range to thetarget. A spatial array of transceivers allows the direction of thereturning wave to be calculated.

The use of sound to measure the depth of the surroundings is attractivebecause the speed of sound is roughly one million times slower than thespeed of light. Therefore, for similar wavelength, resolution, andaccuracy, a system can operate at a frequency ˜1 million times lowerthan that of an optical/RF system. This allows the use of low-speed,power efficient electronics that enables the system to consume orders ofmagnitude less power.

Examples of potential uses for the present invention include, but arenot limited to, the following:

1. Gesture Control:

(a) When non-contact user input is required, such as in a sterileenvironment such as an operating room.

(b) When gesture control is complementary to other input methods, suchas a tablet which has a touch screen input as well as a gesture controlinput. In such a system, gestures might be used to control the macroinputs to the device, such as to advance a slide, scroll, or switchapplications. The touch screen could be used for fine control, such asentering text or navigating menus.

(c) When gesture control is the primary input method:

-   -   (i) Display control.    -   (ii) Vehicle dashboard control.

2. Environment Mapping:

(a) For use on a robotic system or micro-air vehicle to map theenvironment, avoid collisions, etc.

(b) For human monitoring e.g. to detect a driver nodding off at thewheel.

(c) For industrial process control e.g. for tracking the location of anobject without using encoders.

Further aspects of the invention will be brought out in the followingportions of the specification, wherein the detailed description is forthe purpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a schematic diagram of a sensor mounted on a computing deviceand a target such as a hand.

FIG. 2 is a schematic diagram of a sensor array and a target.

FIG. 3A is a cross-section of a single transducer.

FIG. 3B is a cross-section of a multi-layer, micro-machined transducer.

FIG. 4 is a circuit schematic showing a possible embodiment of thesensor system in accordance with the present invention.

FIG. 5 is a micrograph of a seven element linear array of transducers inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally comprises an ultrasonic depth sensorwhich can be used to measure the position and configuration of a user'shand(s) with respect to an electronic device (hereafter “computer”) inorder for the user to generate input into the computer. The sensor maybe configured to detect motions of the user's entire hand or individualfingers. The computer translates the user's motions into input commandsfor the computer, and may generate visual, audio, or tactile feedback tothe user.

The system of the present invention differs from prior approaches inseveral ways. For example, the system of the present invention also usespulse-echo excitation and closely spaced arrays of transducers, wherethe phase and amplitude of the transducers can be controlled to provideangular resolution.

FIG. 1 shows an embodiment of a sensor system according to the presentinvention. Computer 1 has a depth sensor 2 mounted on it such that soundwaves are emitted towards a user's hand 3. Depth sensor 2 transmitssound waves into the air around computer 1. Reflected sound waves from ahand 3 or another object return to depth sensor 2 and the sensordetermines the location of the user's hand 3 with respect to thecomputer 1. Several depth sensors 2 may be used together or separatelyin different locations around the computer. The velocity or accelerationof the user's hand 3 can be determined by the difference in the hand'sposition between measurements. The position, velocity, and/oracceleration (hereafter “state”) of the user's hand 3 can serve as inputto the computer 1. Furthermore, the state of fingers on the hand 3 canalso be measured and be used as input to the computer 1. Also, the stateof a second hand and fingers on a second hand can also be determined asused as inputs to the computer 1. Finally, the state of other objectscan also be used as inputs to the computer 1.

The depth sensor 2 performs a measurement as follows. Sound waves aretransmitted into the air by one or more microelectromechanicaltransmitting elements. The transmitted signal can be either a continuouswave or a pulsed excitation. The sound signals travel into theenvironment and are reflected by any objects present in the environment.The reflected sound signals travel back to one or more receivers. Therange R is estimated from the delay between the transmitted signal andthe received signal according to the speed of sound. The direction tothe target is either estimated from the delay between separate receivedsignals according to the distance between receivers, estimated from thedelay between transmitted signals according to the distance betweentransmitters, or estimated using a combination of both methods.

FIG. 2 shows an array of sound transducers with several transducerelements 6 arrayed on a sensor substrate 4. Sound from transmittingelements 6 is directed towards a target 5 and the reflected signal fromthis target is used to determine the position of the target. Transducerelements 6 may act as transmitters, receivers, or both. The transducersare configured to be smaller than the wavelength of the transmittedsound such that transmit and receive sensitivity is nearly isotropic.The reflected signals are received by the computer 50 having programming54 executable on processor 52 for evaluating the received reflectedsignals to determine the position and/or motion (e.g. gestures) of thehand 3, as will be described in further detail below.

When transmitting a sound wave, one or more transducers may be used. Ifa single transducer is used, the transmitted wave is isotropic.Therefore any reflected signal may originate from objects at a widerange of angles. In this case, an array of transducers is used toreceive the reflected signal, and the target locations are reconstructedby measuring the time delay between the transmitted signal and thereceived signal as well as the time delay between the several receivesignals and the relative location of the receive elements.

If multiple transducers are used to transmit sound waves, a narrow beamis emitted. The direction of this beam can be controlled using phasedarray techniques. In this mode, each transmitter is individuallyaddressed with an electrical signal in order to control the direction ofthe emitted beam. A single receiver may be used to measure the soundwave returning from the direction of the emitted beam, or multiplereceivers may be used in conjunction with multiple transmitters in orderto increase the received signal level and the performance of the system.

If several receivers are used, the received signal level is maximizedwhen the returning signals are added together in a coherent fashion.However, if the returning signals are coming from an object which is ata direction that is not normal to the transducer substrate, they areshifted in time when they arrive at the receive transducers. Thereforethe receiver will ideally introduce a corresponding delay to eachreceive signal in order to maximize the signal level.

In the range axis, range accuracy, range resolution, and the maximum andminimum operating range are important performance metrics. To minimizerange resolution and the minimum operating range and maximize rangeaccuracy, the bandwidth of the transmitted and received signals may bemaximized.

In the angle axis, angular accuracy and angular resolution are theimportant performance metrics. To minimize angular resolution andmaximize angular accuracy, the ratio of the transmitted and receivedsignals may be maximized.

Several transmitter and/or receiver transducers may created on a singlesubstrate. In a preferred sensor configuration 30 shown in FIG. 3A, eachtransducer comprises of a thin membrane 8 that is much thinner than thesubstrate 7 supporting it. The membrane 8 can be actuated usingpiezoelectric transduction, capacitive transduction, thermal actuation,or the like. Actuation causes the membrane 8 to deflect out of the planeof the substrate 7, generating a sound wave. The transducer membrane 8is actuated at its lowest frequency resonance.

As shown in FIG. 3A, substrate 7 supports membrane 8. The membrane 8 canbe released using a tube 9 through the substrate 7. The substrate 7 maybe configured to be approximately an odd multiple of a quarterwavelength of the sound signal, which is the speed of sound divided bythe center of the operating frequency band (λ=c/f). If the substratemeets this condition, the tube 9 on the substrate side of the membrane 8(hereafter ‘back side’) acts as a resonant cavity. This has the effectof increasing the signal level emitted and received by the membrane 8 onthe back side. It also increases the effective damping of the membrane,which increases the bandwidth of the transducer. Furthermore, the use ofan etch through the substrate allows the back side of the substrate 7 tobe facing the air. This serves to protect the thin membrane fromphysical damage.

The specific equation that should be used to choose the thickness of thesubstrate to maximize the effect of the resonant cavity is

$\frac{n\; \lambda}{4} = {L + {0.48\sqrt{A,}}}$

where λ is the wavelength of the sound wave, L is the thickness of thesubstrate, A is the cross-sectional area of the tube, and n is the oddseries n=1, 3, 5 . . . . This equation is applicable when A is largerthan √{square root over (A/π)}.

FIG. 3B shows an embodiment of a piezoelectric Micromachined

Ultrasonic transducer (pMUT) according to the present invention. Thetransducer 40 comprises an AlN/Mo/AlN/AI stack fabricated on a siliconwafer 31 and released with a backside through-silicon etch. In a oneembodiment, the AlN/Mo/AlN/AI stack is approximately 2 μm thick. In thisembodiment, the ultrasound transducer 40 may be in the form of acircular piezoelectric unimorph membrane. Specifically, a bottom layer32 is provided over a substrate 31. The bottom layer 32 may act as apassive bending layer to facilitate out of plane movement, or act as asecond piezoelectric layer. By way of example, but not of limitation,the bottom layer 32 may be made from aluminum nitride (AlN), leadzirconium titanate (PZT), zinc oxide (ZnO), silicon, silicon dioxide,polysilicon, or the like. A bottom metal layer 33 is disposed over thebottom layer 32. In one example, the bottom metal layer 33 may be alayer of molybdenum, aluminum or platinum. A piezoelectric layer 34 issandwiched between the bottom metal layer 33 and the top metal layer 35.The layer 34 may be made of AlN, PZT, ZnO or other piezoelectric. In oneexample, the top metal layer 35 may be made of aluminum, gold (AU), orother metal or combination of metals. The transducer 40 may include anetch hole 36 that exposes the bottom metal layer 33. In one example, theetch hole 36 may be formed by wet or dry etch process. In addition, aresonant tube 37 is formed to define a transducer membrane by deepreactive ion etch on the substrate 31.

Voltage applied across the electrodes results an in-plane stress in thetop AlN layer 34 via the inverse piezoelectric effect. The bottom layerof AlN 32 causes a stress gradient to form across the membrane whichresults in out-of-plane displacement, causing the transducer to emit apressure wave. Similarly, an incident pressure wave causes in-planestress in the top layer of AlN 34, which results in charge developingacross the electrodes that can be sensed electrically.

The 450 μm diameter transducers are designed to have a resonantfrequency f₀≈190 kHz. At atmospheric pressure, the Q of the transduceris about 15, corresponding to a bandwidth BW=f₀/2Q≈6.2 kHz, and themotional impedance of each device is approximately 2 MO at resonance.The effective surface area of each transducer is SA=0.05 mm². Thecapacitance of these devices, including bond pads and interconnect, isapproximately 8 pF.

FIG. 4 shows a schematic diagram of a single channel sensor system 45according to one embodiment of the current invention. In FIG. 4, ameasurement cycle begins when signal generation circuit 10 generates atransmit pulse 25. The transmitting circuit 11 is designed to excite atransducer 13 with a voltage signal near the resonance of the transducer13. The transmitting circuit 11 may include a circuit (not shown) todetermine the resonant frequency of the transducer 13. A switch 12provides isolation between the transmitter 11 and the transducer 13during the time when transducer 13 is not used for transmitting. Thetransmitted signal 25 passes out into the air and may reflect off one ormore targets 5 if they are present. The transmitter 13 stopstransmitting and switch 12 is opened.

Subsequently, switch 15 is closed and the receiving transducer 14 beginsreceiving. The received echo 26 arrives at transducer 14 after a delayaccording to the range to the reflecting surface 5. Amplifier 16amplifies and filters the signal which comprises noise plus any receivedecho signal 26. Analog to digital converter 17 (hereafter “ADC”)converts the analog received signal to a digital form and filters thesignal further. Phase shifter 19 is controlled by phase shift controller18 to delay the signals from ADC 17 by several different amountsaccording to delay vector 24. Signals acquired by other transducers(hereafter ‘other channels’) 20 that have gone through the processingsteps described up to here are added to the signals from phase shifter19 by adders 21 and sent to a target acquisition system 22, which thenoutputs target acquisition data 23.

Each of the delays in delay vector 24 are designed to delay the signalfrom the ADC 17 by an amount corresponding to a direction in space, andaccording to the transducer element 14 location on the sensor assembly.Each delay in delay vector 24 corresponds to a different direction inspace. The range of angles corresponding to delay vector 24 form themeasurement field of view, and each signal leaving adder 21 is anintensity vs. range measurement (hereafter “A-scan”) for a specificdirection of view. If the transmit signal is isotropic, the entire fieldof view can be captured with a single measurement cycle, and the desiredfield of view and angular resolution determines the size of delay vector24, the number of adder blocks 21, and the number of signals processedby target acquisition block 22.

Switch 15 and switch 12 together allow transducer 14 and transducer 13to be the same device (e.g. a transceiver), which is used both totransmit and receive at separate times. When transmitting, switch 12 isclosed and switch 15 is open. When receiving, switch 12 is open andswitch 15 is closed. A single transducer may also be used to transmitand receive simultaneously. In this case switch 12 and switch 15 areclosed.

Target acquisition block 22 thresholds the A-scan signals to determinewhich signals contain echoes. The signals which contain echoes areprocessed to determine the range and direction to the targets 5. Therange to each target 5 is measured by determining the delay between thetransmit pulse 25 and the time when an echo 26 has reached half of itsmaximum amplitude, and using the fact that the range is half themeasured time times the speed of sound. The direction to each target 5is coarsely determined by finding the maximum of the A-scanmeasurements, and using the direction represented by that A-scanmeasurement as the direction to the target 5. A more accurate estimationcan be made by comparing the phase of each received echo signal at theoutput of the ADC 17 to a reference clock. Phase differences betweenchannels are used to determine the direction to each target 5.

The transducer 14 continues to receive data until the elapsed time sincethe transmit pulse 25 began exceeds the twice the maximum desired rangeof the system divided by the speed of sound. A new measurement cycle canbegin at any following time.

FIG. 5 shows an example of a wafer configuration 60 for a spatial arrayof transceivers. Specifically, FIG. 5 shows a micrograph of a sevenelement linear array 62 used to receive echoes from the environment. Toprotect the transducer membranes from damage, the backside of the die 64is exposed to the air. The transmitter 66 is excited with a T_(p)=160μsec burst at f₀ and emits an omnidirectional beam. A transmit signaltwice as long as required by the bandwidth of the transducer is used toallow more accurate measurement of the phase of the return echo. Thisincreases the range resolution from 21 mm to 34 mm. The transmitterpower consumption, based on CV²f losses, is 50 μW. The 1D receiver array66 captures the echoes which are amplified, digitized, and quadraturedown-converted for each channel separately. Digital filtering removeswideband electronic noise. Object positions can be calculated by areal-time digital post-processor. For each angle θ, the signals fromeach channel k are shifted by a phase shift kφ=k2πd/λ sin(θ) and summedwith the other channels. The angle is swept over the entire angle range.Approximate target ranges and angles can be extracted from this data.Subsequently, a fine estimator improves the estimate by searching eachbaseband signal for a pulse in temporal proximity to the coarseestimate. Refined estimates for the range and angle to the target aredetermined from the average time-of-flight for each channel's echo andthe phase difference between adjacent receive channels.

Additional description can be found in R. J. Przybyla et al., “In-airultrasonic range finding and angle estimation using an array of AlNmicromachined transducers,” in Proc. Hilton Head Solid-State Sensors,Actuators and Microsystems Workshop 2012, 3-7 Jun. 2012, pp. 50-53, theentire disclosures of which are incorporated by reference herein.

From the description herein it will be appreciated that the inventioncan be embodied in various ways which include, but are not limited to:

1. An apparatus for determining location of a moveable object inrelation to a computer input device, the apparatus comprising: a depthsensor; said depth sensor comprising an ultrasound emitter; said depthsensor comprising an ultrasound receiver; and means for determininglocation of a moveable object in relation to a computer input deviceusing sound waves that are emitted from the ultrasound emitter,reflected from the moveable object, and received by said ultrasoundreceiver.

2. An apparatus for determining location of a moveable object inrelation to a computer input device, the apparatus comprising: a depthsensor; said depth sensor comprising an ultrasound emitter; said depthsensor comprising an ultrasound receiver; and means for determininglocation of a moveable object in relation to a computer input deviceusing sound waves that are emitted from the ultrasound emitter,reflected from the moveable object, and received by said ultrasoundreceiver; wherein said means for determining location of a moveableobject in relation to a computer input device comprises a processor andprogramming executable on the processor and configured for performingsteps comprising: transmitting sound waves from said ultrasound emitter;wherein said sound waves are continuous or pulsed; receiving sound wavesreflected from the moveable object; calculating delay betweentransmitted and received sound waves; estimating a range R from saiddelay based on the speed of sound; estimating direction to the moveableobject in relation to the depth sensor based on delay between separatereceived signals and distance between multiple ultrasound receivers, orbased on delay between transmitted signals and distance between multipletransmitters, or a combination thereof.

3. The apparatus recited in any preceding embodiment: wherein the depthsensor is an element of an array of depth sensors; and wherein the arraycomprises a plurality of depth sensors on a substrate.

4. The apparatus recited in any preceding embodiment, wherein the depthsensors are smaller in size than the wavelength of sound emitters by thedepth sensors such that transmit and receive sensitivity issubstantially isotropic.

5. An apparatus for determining location of a moveable object inrelation to a computer input device, the apparatus comprising: a depthsensor; said depth sensor comprising an ultrasound emitter; said depthsensor comprising an ultrasound receiver; and means for determininglocation of a moveable object in relation to a computer input deviceusing sound waves that are emitted from the ultrasound emitter,reflected from the moveable object, and received by said ultrasoundreceiver; wherein said means for determining location of a moveableobject in relation to a computer input device comprises: a signalgeneration circuit configured for generating a transmit pulse; atransmitting circuit configured to excite the ultrasound emitter with avoltage signal near the resonance of the ultrasound emitter; a firstswitch configured for providing switchable isolation between thetransmitter circuit and the ultrasound emitter during a time when theultrasound emitter is not used for transmitting; a second switchconfigured for providing switchable isolation of the ultrasound receiverduring a time when the ultrasound is not used for receiving; anamplifier coupled to said ultrasound receiver through said secondswitch, said amplifier configured for amplifying and filtering areceived signal, said received signal comprising noise plus any analogreceived echo signal; an analog to digital converter (ADC) configured toconvert the analog received signal to a digital form and filter thesignal; a phase shifter and phase shifter controller configured to delaysignals from the ADC by a plurality of different amounts according to adelay vector; a plurality of adders configures to add multiple signalsfrom the phase shifter; a target acquisition system configured forprocessing the signals from the phase shifter and generate a useableoutput for controlling the computer.

6. The apparatus recited in an preceding embodiment, wherein each of thedelays in the delay vector configures to delay the signal from the ADCby an amount corresponding to a direction in space, and according to theultrasound receiver location.

7. The apparatus recited in any preceding embodiment, wherein each delayin delay vector 24 corresponds to a different direction in space.

8. The apparatus recited in any preceding embodiment: wherein a range ofangles corresponding to the delay vector form a measurement field ofview; and wherein each signal leaving the adder is an intensity vs.range measurement (A-scan) for a specific direction of view.

9. The apparatus recited in any preceding embodiment, wherein if thetransmit signal is isotropic, the entire field of view can be capturedwith a single measurement cycle, and the desired field of view andangular resolution determines the size of the delay vector, the numberof adders, and the number of signals processed by the target acquisitionsystem.

10. The apparatus recited in any preceding embodiment, wherein thetarget acquisition system is configured to threshold the A-scan signalsto determine which signals contain echoes.

11. The apparatus recited in any preceding embodiment, wherein thesignals which contain echoes are processed to determine the range anddirection to the targets.

12. The apparatus recited in any preceding embodiment, wherein range toeach target is measured by determining the delay between the transmitpulse and the time when an echo has reached half of its maximumamplitude, and using the fact that the range is half the measured timetimes the speed of sound.

13. The apparatus recited in any preceding embodiment, wherein directionto each moveable object is coarsely determined by finding the maximum ofthe A-scan measurements, and using the direction represented by thatA-scan measurement as the direction to the target.

14. The apparatus recited in any preceding embodiment: wherein the phaseof each received echo signal at the output of the ADC is compared to areference clock; and wherein phase differences between channels are usedto determine the direction to each moveable object.

15. An apparatus for determining location of a moveable object inrelation to an input device, the apparatus comprising: an array of oneor more piezoelectric micromachined ultrasonic transducer (pMUT)elements formed from a common substrate, the one or more pMUT elementscomprising one or more transmitters and one or more receivers; and aprocessor configured to determine a location of a moveable object inrelation to an input device using sound waves that are emitted from theone or more transmitters, reflected from the moveable object, andreceived by the one or more receivers.

16. The apparatus of any preceding embodiment: wherein the one or moretransmitters transmit first sound signals and the one or more receiversreceive second sound signals reflected from an movable object; andwherein the processor is configured to calculate the delay between thefirst and the second sound signals, estimate a range R from the delaybased on a speed of sound, estimate a direction of the moveable objectin relation to the input device based on the delay between the secondsound signals received by the one or more receivers and a distancebetween the one or more receivers, or based on a delay between the firstsound signals transmitted by the one or more transmitters and a distancebetween the one or more transmitters, or based on a combination thereof.

17. The apparatus of any preceding embodiment, wherein the one or morepMUT elements are smaller in size than the wavelengths of the firstsound signals such that transmit and receive sensitivity of each elementis substantially omnidirectional.

18. The apparatus of any preceding embodiment: wherein the one or morepMUT elements include a membrane provided over the substrate; whereinthe membrane is thinner than the substrate in thickness; and wherein thesubstrate is etched through to form a cavity under portions of themembrane.

19. The apparatus of any preceding embodiment, wherein a thickness ofthe substrate is configured to maximize an effect of the cavity as aresonant cavity.

20. The apparatus of any preceding embodiment, wherein a back side ofthe substrate opposite the membrane is exposed to air.

21. The apparatus of any preceding embodiment: wherein the membraneincludes a first metal layer sandwiched between a first piezoelectriclayer and a second layer; and wherein the first metal layer, firstpiezoelectric layer, and second layer are configured to form a stressgradient across the membrane which results in an out-of-planedisplacement, causing the pMUT element to emit a pressure wave.

22. The apparatus of any preceding embodiment, wherein the firstpiezoelectric layer is made of aluminum nitride.

23. The apparatus of any preceding embodiment, wherein the second layeris made of aluminum nitride.

24. The apparatus of any preceding embodiment, wherein the first metallayer is made of molybdenum.

25. An apparatus for determining location of a moveable object inrelation to an input device, the apparatus comprising: an array of oneor more piezoelectric micromachined ultrasonic transducer (pMUT)elements formed from a substrate, the one or more pMUT elementscomprising one or more transmitters and one or more receivers; and aprocessor configured to determine a location of a moveable object inrelation to an input device using sound waves that are emitted from theone or more transmitters, reflected from the moveable object, andreceived by the one or more receivers; wherein a transmitter circuitcoupled to one or more transmitters is configured to excite the one ormore transmitters with a voltage signal near a resonance of the one ormore transmitters; wherein one or more amplifiers are coupled to the oneor more receivers respectively, each of the one or more amplifiersconfigured to generate a received analog signal, the received analogsignal comprising noises plus an analog echo signal; wherein one or moreanalog to digital converters coupled to the one or more amplifiersrespectively, each of the one or more analog to digital convertersconfigured to convert the received analog signal and generate a digitalsignal; wherein a phase shifter controller is configured to control oneor more phase shifters coupled to the one or more analog to digitalconverters respectively, each of the one or more phase shiftersconfigured to delay a corresponding digital signal by a plurality ofdifferent amounts according to a delay vector; wherein one or moreadders coupled to the one or more phase shifters respectively areconfigured to add delayed digital signals from the one or more phaseshifters; and wherein a target acquisition system coupled to the one ormore adders is configured to process A-scan signals from the one or moreadders and generate an output for controlling the input device.

26. The apparatus of any preceding embodiment, wherein the transmittercircuit is coupled to one or more transmitters through a first switch,the first switch configured to control connection between thetransmitter circuit and the one or more transmitters.

27. The apparatus of any preceding embodiment, wherein the one or moreamplifiers are coupled to the one or more receivers respectively throughcorresponding second switches, each of the second switches configured tocontrol connection between the corresponding amplifier and the receiver.

28. The apparatus of any preceding embodiment, wherein each of the oneor more phase shifters is configured to delay a corresponding digitalsignal by a plurality of different amounts corresponding to a directionin space, and according to locations of the one or more receivers.

29. The apparatus of any preceding embodiment, wherein the targetacquisition system is configured to threshold the A-scan signals todetermine which signals contain echoes.

30. The apparatus of any preceding embodiment, wherein the signals whichcontain echoes are processed to determine a range and direction of themovable object in relation to the input device.

31. A method for determining location of a moveable object in relationto an input device, the method comprising: transmitting a first soundsignal from an array of one or more transceivers;

receiving a second sound signal reflected from a movable object inproximity to the array; and

determining a location of a moveable object in relation to an inputdevice as a function of the second sound signal.

32. The method of any preceding embodiment, wherein determining alocation of a moveable object in relation to an input device furthercomprises: calculating a delay between the first and the second soundsignals; estimating a range R from the delay based on a speed of sound;and estimating a direction of the moveable object in relation to theinput device.

33. The method of any preceding embodiment, wherein estimating adirection of the moveable object in relation to the input device is afunction of one or more of: a delay between the second sound signalsreceived by the one or more transceivers and a distance between the oneor more transceivers, or a delay between the first sound signalstransmitted by the one or more transceivers and a distance between theone or more transceivers.

34. An apparatus for determining location of a moveable object inrelation to an input device, the apparatus comprising: an array of oneor more piezoelectric micromachined ultrasonic transducer (pMUT)elements formed from a common substrate, the one or more pMUT elementscomprising one or more transceivers; a processor; and programmingexecutable on the processor to determine a location of a moveable objectin relation to an input device using sound waves that are emitted fromthe one or more transceivers, reflected from the moveable object, andreceived by the one or more transceivers.

Embodiments of the present invention may be described with reference toflowchart illustrations of methods and systems according to embodimentsof the invention, and/or algorithms, formulae, or other computationaldepictions, which may also be implemented as computer program products.In this regard, each block or step of a flowchart, and combinations ofblocks (and/or steps) in a flowchart, algorithm, formula, orcomputational depiction can be implemented by various means, such ashardware, firmware, and/or software including one or more computerprogram instructions embodied in computer-readable program code logic.As will be appreciated, any such computer program instructions may beloaded onto a computer, including without limitation a general purposecomputer or special purpose computer, or other programmable processingapparatus to produce a machine, such that the computer programinstructions which execute on the computer or other programmableprocessing apparatus create means for implementing the functionsspecified in the block(s) of the flowchart(s).

Accordingly, blocks of the flowcharts, algorithms, formulae, orcomputational depictions support combinations of means for performingthe specified functions, combinations of steps for performing thespecified functions, and computer program instructions, such as embodiedin computer-readable program code logic means, for performing thespecified functions. It will also be understood that each block of theflowchart illustrations, algorithms, formulae, or computationaldepictions and combinations thereof described herein, can be implementedby special purpose hardware-based computer systems which perform thespecified functions or steps, or combinations of special purposehardware and computer-readable program code logic means.

Furthermore, these computer program instructions, such as embodied incomputer-readable program code logic, may also be stored in acomputer-readable memory that can direct a computer or otherprogrammable processing apparatus to function in a particular manner,such that the instructions stored in the computer-readable memoryproduce an article of manufacture including instruction means whichimplement the function specified in the block(s) of the flowchart(s).The computer program instructions may also be loaded onto a computer orother programmable processing apparatus to cause a series of operationalsteps to be performed on the computer or other programmable processingapparatus to produce a computer-implemented process such that theinstructions which execute on the computer or other programmableprocessing apparatus provide steps for implementing the functionsspecified in the block(s) of the flowchart(s), algorithm(s), formula(e),or computational depiction(s).

Although the description herein contains many details, these should notbe construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Therefore, it will be appreciated that the scope ofthe present invention fully encompasses other embodiments which maybecome obvious to those skilled in the art.

Although the description above contains many details, these should notbe construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Therefore, it will be appreciated that the scope ofthe present invention fully encompasses other embodiments which maybecome obvious to those skilled in the art, and that the scope of thepresent invention is accordingly to be limited by nothing other than theappended claims, in which reference to an element in the singular is notintended to mean “one and only one” unless explicitly so stated, butrather “one or more.” All structural, chemical, and functionalequivalents to the elements of the above-described preferred embodimentthat are known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe present claims. Moreover, it is not necessary for a device or methodto address each and every problem sought to be solved by the presentinvention, for it to be encompassed by the present claims. Furthermore,no element, component, or method step in the present disclosure isintended to be dedicated to the public regardless of whether theelement, component, or method step is explicitly recited in the claims.No claim element herein is to be construed under the provisions of 35U.S.C. 112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for.” Any element in a claim that does notexplicitly state “means for” performing a specified function, is not tobe interpreted as a “means” or “step” clause as specified in 35 USC§112, sixth paragraph. In particular, the use of “step of” in the claimsherein is not intended to invoke the provisions of 35 USC §112, sixthparagraph.

What is claimed is:
 1. An apparatus for determining location of amoveable object in relation to an input device, the apparatuscomprising: two or more piezoelectric micromachined ultrasonictransducer (pMUT) elements, the two or more pMUT elements comprising oneor more transmitters and two or more receivers; and a processorconfigured to determine a location of a moveable object in relation toan input device using sound waves that are emitted from the one or moretransmitters, reflected from the moveable object, and received by thetwo or more receivers; and wherein each of the two or more receivers iscoupled to two or more receive amplifiers, each of the two or morereceive amplifiers configured to generate a received analog signal;wherein two or more analog to digital converters coupled to the two ormore amplifiers respectively, each of the two or more analog to digitalconverters configured to convert the received analog signal and generatea digital signal; and wherein each of the one or more transmitters iscoupled to one or more transmitter circuits configured to excite the oneor more transmitters with a voltage signal near a resonance of the oneor more transmitters; and wherein the one or more transmitters have anemission pattern that is substantially isotropic so that the transmittedsound covers a wide range of angles.
 2. The apparatus of claim 1,wherein the signals received from the one or more receive amplifiers aredown-converted from a transmit frequency to a baseband frequency using aquadrature demodulator.
 3. The apparatus of claim 2, wherein thebaseband signals are subsequently processed to determine a time ofarrival of the second sound signals received by the two or morereceivers.
 4. The apparatus of claim 3, wherein the processor isconfigured to determine a range R and direction of the moveable objectfrom a speed of sound, the time of arrival of the second sound signalsreceived by the two or more receivers, and a distance between the two ormore receivers and the one or more transmitters.
 5. The apparatus ofclaim 1, wherein one or more of the pMUT elements functions as both atransmitter and a receiver.
 6. The apparatus of claim 5, wherein each ofthe one or more pMUT elements is coupled to a transmitter circuitthrough a first switch and is coupled to a receive amplifier through asecond switch.
 7. The apparatus of claim 6, wherein the signals receivedfrom the one or more receive amplifiers are down-converted from atransmit frequency to a baseband frequency using a quadraturedemodulator.
 8. The apparatus of claim 7, wherein the baseband signalsare subsequently processed to determine a time of arrival of the secondsound signals received by the two or more receivers.
 9. The apparatus ofclaim 8, wherein the processor is configured to determine a range R anddirection of the moveable object from a speed of sound, the time ofarrival of the second sound signals received by the two or morereceivers, and a distance between the two or more receivers and the oneor more transmitters.
 10. The apparatus of claim 7: wherein the basebandsignals from the one or more receivers are subsequently phase shiftedusing one or more phase shifters configured to delay each of thebaseband signals by a plurality of different amounts according to adelay vector; and wherein one or more adders coupled to the one or morephase shifters respectively are configured to add delayed digitalsignals from the one or more phase shifters, the resulting output ofeach of the one or more adders comprising an intensity vs. rangemeasurement (“A-scan”) corresponding to a specific view angle.
 11. Theapparatus of claim 10 wherein the range R and direction of the moveableobject in relation to an input device are determined by processing theA-scan signals.